1. Field of the Invention
The present invention relates generally to battery charging systems and more specifically to battery chargers with self contained power sources.
2. Description of the Related Art
Typical battery-powered systems utilize an external or internal battery charging circuit. The power source to the battery charging circuit may be provided through an AC to DC converter (AC adaptor) that converts a commercially available AC power (AC mains) to a low voltage DC voltage. In addition, the DC voltage can also be used to power downstream electronics directly, which is otherwise powered by the battery when the AC adaptor is not present.
One example is a mobile handset powered by a single cell Lithium-Ion battery with a 3.6V nominal output voltage. The battery is typically charged by an AC adaptor, through a battery charging circuit. The battery charging circuit can be either a semiconductor based integrated circuit (IC) or a circuit built with discrete components. Also, within the mobile handset, there are usually 3 to 5 voltage rails, such as 5V, 3.3V, 2.5V, 1.8V, and 1.5V, etc., that are used to power other functional blocks or circuits providing specific functions for the mobile handset. These voltage rails can either be converted from the Lithium-Ion battery when an AC adaptor is not present, or converted from the DC voltage output of the AC adaptor when the AC adaptor is present.
As such, a power manager in a battery-powered system includes a battery charging circuit and a power conversion circuit. The battery charging circuit, in which power is supplied through an AC adaptor can be a circuit built with discrete components, or an IC, or a part of an IC. The power conversion circuit converts either the battery voltage or the AC adaptor provided voltage to a variety of other voltages that supply other circuits of the system. The power conversion circuit can be a circuit built with discrete components, or a collection of ICs, or an IC, or a part of an IC.
With the DC output voltage of the AC adaptor being higher than that of the battery voltage, the battery charging circuit is typically of a step-down topology, such as a linear or “buck” converter, which converts a higher DC voltage to a lower one.
For the power conversion circuit, in which the voltages utilized are higher than the nominal output voltage of the battery, a step-up or “boost” topology is used and for voltages that are lower than the nominal output voltage of the battery, a step-down or “buck” topology is used. As such, for a power conversion circuit, both step-up and step-down sub-circuits are used. These sub-circuits are typically provided as separate or independent ICs.
As such, on a system level, an AC adaptor, or other typical or external power source, is always needed. In addition, the DC output voltage of the AC adaptor needs to be above a certain level. This minimum voltage is typically between 0.7V and 1.0V, depending upon the threshold voltage of the switching device (MOSFET, bipolar transistor, etc.) used in the voltage converter. This means the DC output of the AC adaptor has to be at least 0.7V to 1.0V. In practice, the DC output of the AC adaptor is almost always higher than the battery voltage so that an easier to implement linear or buck topology can be used for the battery charging circuit. For example, the AC adaptor for a single-cell Lithium-Ion battery (3.6V nominal voltage) powered system is typically higher than 5V.
As a result, the power source is not “self-contained”, i.e., power is drawn from an external power source, such as the AC mains, which greatly reduces the mobility of certain applications such as mobile handsets.
Thus, it is desirable to have a power source that is completely “self-contained”. It is also desirable to include a “renewable” voltage generator that can provide power to the battery charging circuit and the power conversion circuit.